In telecommunication systems, it is known that not only the subscriber instrument itself but also auxiliary equipment may be supplied with current by way of the subscriber line. Besides information signals, that is, a direct current supply for a terminal is transmitted by way of the subscriber line as well, for which purpose special disconnection circuits are provided at the beginning and end of a subscriber line to cut the information signals in and out respectively.
In modern telecommunication systems, and particularly digitally operating telephone extension systems, use is increasingly being made of intelligent terminals, equipped with microprocessors, at the subscriber connections. Examples of such terminals are digital telephone subscriber stations, data printers, monitors, etc. These terminals have heretofore been supplied directly from the power supply network, since their operation requires a preassigned supply voltage which is subject as little as possible to fluctuation in order to ensure proper function or stable operation. Remote supply of such terminals by way of the subscriber line is not consistent wiht prevailing concepts of circuitry, because, due to the electrical properties of the lines used in telecommunication systems, fluctuations in current and/or voltage, especially when such a supply system is connected to the central source of current, may lead to uncontrolled switch-on and switch-off processes which firstly may cause errors in the transmission and evaluation of the information signals and secondly, due to sharply fluctuating supply currents, may produce adverse side effects on other components of the telecommunication system, which may likewise lead to uncontrolled operating conditions.
If it is desired nevertheless to achieve remote supply of subscriber line terminals by way of the subscriber line, requirements must be met that result from the special employment of power supply by way of the subscriber line. A first requirement consists, in view of the limited load capacity of a subscriber line, of keeping the tension of a supply current carried by it comparatively low, for which purpose the system as a whole must have an optimal efficiency. At the same time, however, both the short circuit capability and the overload capability of the system must be assured, since in telecommunication systems both operating conditions may occur often because of the high line resistances compared to network power supply. These line resistances result in excessively strong switch-on currents on the subscriber line when a terminal is connected to it, producing such high voltage drops on it that the connected terminal will disconnect itself from the subscriber line again due to insufficient operating voltage. Hence, means should be provided to avoid such switching conditions. Finally, any ground loops in the system must also be avoided, since they may cause impairment of information signal transmission.